(1) Field of the Invention
The present invention relates to an electrophotographic element, in particular relates to an electrophotographic element comprising a layer (which is called a charge carrier generating layer hereinafter) containing a substance capable of generating charge carriers when exposed to light radiation (which is called a charge carrier generating substance hereinafter) and a layer (which is called a charge transfer layer hereinafter) containing a substance capable of accepting the charge carriers generated by said charge carrier generating layer and transferring them (which is called a charge transfer substance hereinafter).
(2) Description of the Prior Art
As typical conventional electrophotographic elements there can be enumerated inorganic and organic ones, the former including those prepared by using selenium or its alloys or dispersing color-sensitized zinc oxides in binder resins, the latter including those prepared by using charge transfer complexes composed of 2,4,7-trinitro-9-fluorenone (which is called TNF hereinafter) and poly-N-vinylcarbazole (which is called PVK hereinafter). However, the fact is that these elements involve various advantages as well as various disadvantages. For instance, now widely used selenium elements are defective in that production cost is high due to complicated manufacturing conditions, it is hard to process them into belts due to lack of flexibility, and they must be handled carefully due to high sensitivity to heat or mechanical impact. On the other hand, zinc oxide elements are advantageous in that their production cost is low because they can be manufactured by coating inexpensive zinc oxides onto substrates, but said zinc oxide elements are generally low in sensitivity, lack surface smoothness and are inferior in mechanical characteristics such as hardness, tensile strength and friction resistance. Accordingly, those zinc oxide elements still involve problems of durability and so forth that need to be solved in order that they may be used as elements for use in plain paper copying machines which are generally run repeatedly. In addition, elements using charge transfer complexes composed of TNF and PVK are unsuitable to be used as elements for use in high speed copying machines because of low sensitivity.
Recently, an extensive study has been carried out to eliminate drawbacks inherent in these elements, in particular various kinds of organic elements have been proposed. Among them, public attention has been attracted to such laminate type elements, prepared by forming an organic dye thin film (charge carrier generating layer) on an electrically conductive substrate and further forming thereon a layer consisting essentially of a charge transfer substance (charge transfer layer), as elements for use in plain paper copying machines because they are generally of high sensitivity and stable chargeability compared with conventional organic type elements. Some of them are put to practical use.
The electrostatic latent image forming mechanism using this kind of laminate type element is considered to consist in that in case the element is charged and then exposed to light radiation, the light penetrates through the transparent charge transfer layer and is absorbed by the charge carrier generating substance incorporated in the charge carrier generating layer, the charge carrier generating substance, upon absorbing the light, generates charge carriers, these charge carriers are injected in the charge transfer layer and travel within the charge transfer layer along the electric field caused by the electric charge, and thus the electric charge on the element surface is neutralized, whereby an electrostatic latent image is formed. Accordingly, the charge carrier generating substance used in this kind of element is required to be one capable of generating charge carriers with high efficiency when exposed to light radiation for the image forming purpose.
On the other hand, the charge transfer substance is required to be transparent to the light used and capable of maintaining a desired charged potential and transferring the charge carriers generated by the charge carrier generating substance rapidly when exposed to light radiation.
As the conventional laminate type elements of this kind there can be enumerated (1) the element using the charge carrier generating layer incorporating a perylene derivative therein and the charge transfer layer incorporating an oxadiazole derivative therein (which see U.S. Pat. No. 3,871,882), (2) the element using the charge carrier generating layer formed by coating Chlorodiane Blue with an organic amine as the solvent and the charge transfer layer incorporating a pyrazoline derivative therein (which see Japanese Laid Open Patent Application 55643/1977 and Japanese Laid Open Patent Application 72231/1977), (3) the element using the charge carrier generating layer formed by coating a dispersion obtained by dispersing a triphenylamine type trisazo pigment (which see Japanese Laid Open Patent Application 132347/1978) for instance in a dispersion medium such, for instance, as tetrahydrofuran or the like and the charge transfer layer incorporating 2,5-bis(4-diethylaminophenyl)-1,3,4-oxadiazole or TNF, and the like.
However, the fact is that the conventional laminate type elements of this kind possess a number of advantages and simultaneously various drawbacks.
For instance, the element using perylene and oxadiazole derivatives as shown in the preceding (1) does not cause any problem when put to practical use, but is inferior in sensitivity when used in high speed copying machines. And, this element is also defective in that it is not used suitably in color copying machines because the charge carrier generating substance on which the spectral sensitivity depends, namely the perylene derivative does not have absorption over the whole visible area.
The element using Chlorodiane Blue and a pyrazoline derivative as shown in the preceding (2) is noted, from our inventors' experiment, to have a relatively good sensitivity but involve many defects that there is necessity of using the organic amine being generally difficult to handle (for instance, ethylenediamine) during its preparation and the like.
The element shown in the preceding (3), which has been proposed by our inventors, is advantageous in that the charge carrier generating layer can be formed easily by coating the substrate with a pigment dispersion obtained by dispersing fine pigment particles in the organic solvent (a binder resin may be added thereto as occasion demands), but is defective in that this element is somewhat inferior in sensitivity and so can not suitably used in high speed copying machines. Of late years, the demand for elements for use in laser printers has been growing, and the development of elements capable of exhibiting high sensitivity especially to the wavelength region of semi-conductor laser has been demanded strongly. However, the fact is that the above mentioned elements are extremely low-sensitive to these semi-conductor lasers and so can not be put to practical use.